The present invention relates to a process of making a cylindrical slotted sieve for strainers and classifiers of paper pulp obtained from used paper, and the sieve manufactured by this process.
In the manufacture of paper pulp from used paper numerous sieves are used both for removing the impurities and for classifying the fibers according to their length.
These sieves are generally perforated cylinders mounted fixed in an enclosed space having at least one inlet of dirty pulp, one outlet of sifted pulp, and one outlet for the waste. The pulp is agitated by one or several vanes which rotate at great speed in the immediate vicinity of the perforated surface.
According to the characteristics of the pulp to be strained (for example the length of the fibers, the nature and size of the contaminants), one uses sieves with holes or with slots. In addition these perforations are usually accompanied by an obstacle which produces pulsations favoring the sifting.
The perforations are normally made by machining sheetmetal of a thickness of about one centimeter. But as this machining presents great difficulties, the applicant has made a sieve from a sheetmetal of thin thickness, of the order of 2 millimeters described in EP-A-O 0 354,846. This sieve has U-sections joined side by side and their bottom is perforated. The perforations are also obtained by machining (sawing, drilling) and the thinness of the sheetmetal greatly simplifies these operations.
However, this technique of making cylindrical sieves of thin sheetmetal still presents difficulties:
when the cylinder is obtained by juxtaposition of arched, i.e., U-shaped, sections, the sections are joined and held tightly together by welding of the adjacent walls or by placing a rider which pinches the two walls. In practice this connection can be made only when the walls of the U-shaped sections are turned toward the outside of the cylinder; PA1 when it is provided that the sifting openings are associated with an obstacle, the machining is such that it is always situated on the outward side of the bottom of the U-shaped section. PA1 the pricking is done on the external face of the bottom of the U toward the interior. As the surface of the sieve is defined by the external face of the bottom of the U, this surface then presents a plurality of perforated hollows: PA1 the distance between two successive pricks is small so that, considered in the direction of flow of the pulp, the inclined face of a deformation is immediately followed by the vertical wall, the profile being then in sawtooth form. PA1 the pricking is done on the inner face of the bottom of the U toward the exterior. Here, the surface of the sieve then presents a plurality of obstacles in relief; PA1 the pricking pattern is determined so that the adjacent and opposite lateral ends of two adjacent obstacles form, when the U-shaped elements are juxtaposed, a groove of approximately concave form.
As the walls of the U-shaped sections are turned toward the outside of the cylinder, this practically makes only the production of centrifugal sieves possible.